Evenly admixing fuel into the combustion air is the central design concern in the development of burners which are operated in the range of what is termed lean premix combustion. The temperature range for lean premix combustion is particularly suitable for gas turbines because of the material-imposed restriction on the combustion chamber outlet temperature for the purpose of controlling nitrous oxide emissions. The difficulty with premix combustion, in particular under pressure, is the avoidance of uncontrollable combustion events/auto-ignition within the premix path, which are generally associated with destruction of the premix path.
As a rule of thumb, it can be assumed that a premix which is as homogeneous as possible also results in minimal NOx emissions.
In general, technical burners are of approximately rotationally symmetric construction and often have one or more swirl generators arranged concentrically around each other. The current prior art is to embody the swirl generator blades as hollow blades and simultaneously to use them as fuel injection elements, see for example WO 2011/023669. Such arrangements generally mean that there is a marked difference in the spacing between two adjacent blades at the hub compared to the outer section, i.e. the blades are generally much closer together at the hub than in their outer region. This gives rise to the problem of injecting the fuel sufficiently far, in particular in the radially outer region, into the interspace between two blades, in order to achieve the best possible mixing, while this is substantially easier to achieve at the hub side.
There have been suggestions for how to solve this problem. In general, however, the improvement is not enormous.
For example:                Increasing the fuel bore diameter from the inner regions to the outer regions. This does result in a partial—although not entirely sufficient—improvement in the penetration depth of the fuel jets, although at the same time the required mixing path is increased (approximately proportional to the diameter of the fuel bore).        Increasing the number of blades. This faces structural and aerodynamic limitations due to the available space on the hub and the increasing blocking of the air flow on the hub side.        Using strongly 3-dimensional blade profiles whose blade thickness increases with increasing distance from the hub. Here, too, problems arise with respect to the aerodynamic contour of the blade, this time in the outer section, in order to go from the large blade thickness in the plane of the fuel bores back to a very thin blade end, i.e. the outer portion of the blades would then be very long.        